When you burn a gallon of gas [petrol in UK English] in your car, you emit about 5 pounds (2.3 kg) of carbon into the atmosphere. If it were solid carbon, it would be extremely noticeable -- it would be like throwing a 5-pound bag of sugar out the window of your car for every gallon of gas burned. But because the 5 pounds of carbon comes out as an invisible gas, carbon dioxide, most of us are oblivious to it. The carbon dioxide coming out of every car's tailpipe is a greenhouse gas that is slowly raising the temperature of the planet. The ultimate effects are unknown, but it is a strong possibility that, eventually, there will be dramatic climate changes that affect everyone on the planet. For example, if the ice caps melt, sea level will rise significantly, flooding and destroying all coastal cities in existence today. That's a big side effect.